1. Field of the Invention
The present invention is in the field of systems for testing water chemistry, and, more particularly, is in the field of amperometric sensors.
2. Description of the Related Art
Amperometric chlorine sensors are used in measuring chlorine residuals in drinking water, wastewater, cooling towers. Some newer applications are measurement of Total Residual Oxidant (TRO) in seawater. Recent regulatory actions require treatment of ballast water onboard ships to inactivate invasive species and prevent their discharge into non-native waters. Another new application is the measurement of up 500 ppm TRO in seawater used for biofouling back flushes of pretreatment microfiltration membranes used in Reverse Osmosis Desalination Systems.
A common problem encountered with online measurement of water chemistry in the field is fouled electrodes in the sensor system. Electrode measurements can be rendered unreliable when the working electrode is covered with either inorganic (salts such as calcium carbonate) layers or organic (biofouling) layers that inhibit electrode processes.
Another problem with certain sensors is the lack of a flow independent measurement method that can installed in a process flow without adverse effects from changing flow rates on the sensor signal. Amperometric sensors, in particular, are significantly affected by flow rates. Preferably, a sensor system should provide accurate measurements with flow rates ranging from 0 to more than 7 feet per second without an appreciable change in the sensor output signal. Another limitation of most sensors is the lack of an ability to directly insert a sensor into a process flow pipe or fitting for a simple installation such as may be necessary in a drinking water distribution system.
Maintenance costs of online chlorine monitors often greatly exceed the cost of the unit. Frequent recalibration is necessary with most, if not all, commercially available sensors due to changing electrode surface, fouling, electrolyte depletion, membrane fouling or stretching, pressure changes or spikes, flow changes and changes in pH.
Many sensor systems require a reagent feed of either an iodide solution or a buffer to lower the pH to 4.0. Other systems are dependent on flow or pressure and require controlled flow with a drain to waste to provide a constant flow rate to the sensor. This requirement further complicates the installation, maintenance and logistical requirements. This results in excessive water loss or unnecessary consumption. Solution consumption and replenishment result in higher costs and maintenance requirements.
Polarization of many sensors causes a loss of sensitivity over the first 2-24 hours, requiring recalibration. Sensitivity is reduced and the calibration changes if the sensor is removed and replaced.
When amperometric systems with exposed electrodes are used with even low levels of cyanuric acid (CYA), a polarization of the electrode occurs with a layer of CYA that inhibits electron processes, rendering the electrode unusable after less than one day of operation due to the low signal response.
Oxidation reduction potential (ORP) is often used for residual control and provides a qualitative indicator of sanitizer efficacy. The probes used for ORP sensing often suffer from a reduction in sensitivity caused by organics, particularly when high levels of cyanuric acid are present. An ORP sensor is needed that does not have this limitation.
The vast majority of commercial amperometric systems use membranes and electrolyte to control the reaction that occurs at the working (measurement) electrode. Problems with these systems are well known. Membranes stretch and foul with oil and organics and must be replaced frequently. Another problem with most of these systems is that they measure only hypochlorous acid, a free chlorine fraction, not free chlorine as measured by a DPD test kit. As a result, membrane sensors' signals change radically with a change in pH compared to bare electrode sensors. This results in a limited operational range of 6.0 to 8.0 pH. Beyond a pH of 8.0, the sensors produce very little signal, which can produce large errors.
Electrolyte must be replaced on an ongoing basis, often monthly. In addition to these deficiencies, membrane sensors are affected by flow and pressure changes requiring recalibration when either changes.
A known sensor uses a replaceable thin-film sensor formed on a substrate with multiple electrodes and a screen printed membrane on the working electrode for chlorine. The sensor has a very short life of approximately 6 months.
Another issue with known sensors is the detection and/or interference of chloramines in drinking water. Chloramine (monochloramine) is a disinfectant used to treat drinking water. Chloramine is most commonly formed when ammonia is added to chlorine to treat drinking water. The typical purpose of chloramine is to provide a longer-lasting residual for disinfection as the water moves through pipes to consumers. This type of disinfection is known as secondary disinfection. Chloramine has been used by water utilities for almost 90 years, and the use of chloramine is closely regulated. More than one in five Americans uses drinking water treated with chloramine. Water that contains chloramine and that meets EPA regulatory standards is safe to use for drinking, cooking, bathing and other household uses.
Many utilities use chlorine as a secondary disinfectant; however, in recent years, some utilities have changed the secondary disinfectant to monochloramine to meet disinfection byproduct regulations.
Monochloramine (NH2Cl) is commonly used in low concentrations as a secondary disinfectant in municipal water distribution systems as an alternative to free chlorine chlorination. The use of monochloramine is increasing. Chlorine (sometimes referred to as “free chlorine”) is being displaced by monochloramine, which is much more stable and does not dissipate from the water before it reaches consumers. Monochloramine also has a very much lower, however still present, tendency than free chlorine to convert organic materials into chlorocarbons such as chloroform. Such compounds have been identified as carcinogens; and in 1979 the United States Environmental Protection Agency began regulating levels of such compounds in drinking water. Furthermore, water treated with chloramine lacks the distinct chlorine odor of the gaseous treatment and so has improved taste. In swimming pools, chloramines are formed by the reaction of free chlorine with organic substances. Chloramines, compared to free chlorine, are both less effective as a sanitizer and more irritating to the eyes of swimmers. When swimmers complain of eye irritation from “too much chlorine” in a pool, the problem is typically a high level of chloramines. Some pool test kits designed for use by homeowners are sensitive to both free chlorine and chloramines, which can be misleading.
The following chart illustrates the current versus the Cl potential and versus the monochloramine potential that is used to determine which species is present:
Analytical Signal Analytical Signal (μA)(μA) (corr.)Potential (V)ChlorineBlankNH2ClBlankChlorineNH2Cl0.301.1790.0160.0340.0081.1620.0260.201.4000.0160.0090.0041.3830.0050.101.65950.0270.2480.0281.6330.2200.002.2870.2410.7450.3112.0460.434
In the foregoing chart, the concentration of Cl is 20 parts per million (ppm), and the concentration of NH2Cl is 20.5 ppm as Cl. The “blank” readings in the two columns are determined by measuring a known liquid with 0 concentration of chlorine or monochloramine. The corrected analytical signals for chlorine and monochloramine are determined by subtracting the blank values from the measured values at each applied potential. The data points for the corrected analytical signals versus the potential voltage are plotted in the graph shown in FIG. 40.
The following chart illustrates the ratios of the analytical signals for corresponding ratios of the potentials:
Ratios of Corrected Analytical Signals (μA) versus ratios of potentials (V)Ratios (V)Chlorine (corrected)NH2Cl (corrected)0.10:0.301.408.5780.00:0.301.7616.913
As can be seen from the foregoing data, it is possible to determine whether the species is monochloramine or free chlorine by the ratio of the measurement of two potentials. A ratio of greater than 5 indicates that the species is monochloramine. A display icon can be used to indicate that monochloramine is present to thereby invite the use to perform an action. For example, when used with a swimming pool, the icon may warn the operator that the water needs to be superchlorinated or that a non-chlorine shock compound needs to be added to lower the monochloramine level.
The data also illustrates the dramatic difference in signals produced by similar levels of free chlorine and monochloramine. Two stored calibrations can be used—one for free chlorine and one for total chlorine. The ratio can also be used to quantify the monochloramine and free chlorine fractions based on the magnitude of the ratio. The stored calibrations can be adjusted to read out in parts per million (ppm) of monochloramine if monochloramine is present. In a drinking water application, the residual displayed can be adjusted to display ppm monochloramine instead of free chlorine to more accurately display the chlorine level. Otherwise, the sensor system may drastically under report the sanitizer levels due to the lower signal response of monochloramine.